Traumatic, inflammatory, and degenerative disorders of the spine can lead to severe pain and loss of mobility. For example, back and spinal musculoskeletal impairments are significant causes of lost work productivity in the United States. Pain as a result of some type of spinal impairment may have its source in a variety of pathologies or clinical conditions.
As shown in FIG. 1, the vertebral column 2 of the spine includes a series of alternating vertebrae 4 and fibrous discs 6 that provide axial support and movement to the upper portions of the body. The vertebral column 2 typically includes thirty-three vertebrae 4, with seven cervical (C1-C7), twelve thoracic (T1-T12), five lumbar (L1-15), five fused sacral (S1-S5) and four fused coccygeal vertebrae.
FIGS. 2A and 2B depict a typical thoracic vertebra. Each vertebra includes an anterior body 8 with a posterior arch 10. The posterior arch 10 has two pedicles 12 and two laminae 14 that join posteriorly to form a spinous process 16. Projecting from each side of the posterior arch 10 is a transverse 18, superior 20 and inferior articular process 22. The facets 24, 26 of the superior 20 and inferior articular processes 22 form facet joints 28 where they align with the articular processes of the adjacent vertebrae, as shown in FIGS. 3A and 3B. The facet joints are true synovial joints, with cartilaginous surfaces and a joint capsule.
The orientation of the facet joints vary, depending on the level of the vertebral column. FIGS. 4A to 6B depict the orientations of the facet joints at different levels of the vertebral column. In the C1 and C2 vertebrae (not shown), the facet joints are substantially parallel to the transverse plane.
In the C3 to C7 vertebrae shown in FIGS. 4A and 4B, the facets are oriented at an approximately 45-degree angle to the transverse plane 30 and are substantially parallel to the frontal plane 32. This orientation allows the facet joints of the cervical vertebrae to flex, extend, laterally flex, and rotate. The 45-degree angle orientation with respect to the transverse plane 30 allows the facet joints of the cervical spine to guide the movement of the cervical vertebrae without limiting such movement.
FIGS. 5A and 5B depict the thoracic vertebrae, which include facets oriented at an approximately 60-degree angle to the transverse plane 30 and an approximately 20-degree angle to the frontal plane 32. This orientation is capable of allowing lateral flexion and rotation, but only limited flexion and extension.
FIGS. 6A and 6B illustrate the lumbar region, where the facet joints are oriented at approximately 90-degree angles to the transverse plane 30 and an approximately 45-degree angle to the frontal plane 32. The lumbar vertebrae allow flexion, extension and lateral flexion of the lumbar region, but little, if any, rotation because of the 90-degree orientation of the facet joints in the transverse plane. The actual range of motion along the vertebral column can vary considerably with each individual vertebra.
In addition to guiding movement of the vertebrae, the facet joints also contribute to the load-bearing ability of the vertebral column. For example, facet joints can bear up to 30% of the load on the spine in some positions of the vertebral column as described, e.g., in King et al., Mechanism of Spinal Injury Due to Caudocephalad Acceleration, Orthop. Clin. North Am., 6:19 (1975). The facet joints may also play a role in resisting shear stresses between the vertebrae. Over time, these forces acting on the facet joints can cause degeneration and arthritis.
One source of back and spine pain is related to degeneration of the facets of the spine or facet arthritis. Bony contact or grinding of degenerated facet joint surfaces between two vertebrae may play a role in some pain syndromes. Such degenerative problems with the facet joints are often treated by fusing the two adjacent vertebrae together. By performing this surgical procedure, the relative motion between the two adjacent vertebrae is effectively stopped. This fusion procedure significantly reduces relative motion of the facets between the adjacent vertebrae. However, the facets between fused vertebrae may still exhibit some relative motion as the vertebral column is subjected to overall movement and various stresses. Such motion can lead to further problems, such as pain, arising from the degenerative facet joint.
Often, the facet joints between fused vertebrae are not treated as part of the fusion procedure. In certain procedures, the facets may simply be screwed together. However, because there is cartilage in the facet joints, the joints may not fuse and can still be a source of further discomfort.
Accordingly, there is a need to address the clinical concerns raised by degenerative facet joints, and to immobilize such facet joints when adjacent vertebrae are fused together.